1. Field of the Invention
The present invention relates to image forming apparatuses, such as copying machines, facsimile machines and printers, that have a printer engine to form an image by scanning a laser beam light using a rotating polygonal deflector deflecting the laser beam light and a motor rotating the polygonal deflector. More particularly, the present invention relates to an image forming apparatus and method to shorten the printing time for a first print sheet by shortening the time a motor rotating a polygonal deflector takes to revolve at a predetermined rate from a rest state.
2. Discussion of the Background
A laser beam printer is a known image forming apparatus having a printer engine for forming an image by scanning a laser beam light using a rotating polygonal deflector deflecting the laser beam light and a motor rotating the polygonal deflector (hereinafter polygon motor). The laser beam printer writes an image on a photoconductor by scanning the laser beam in the main scanning direction with rotation of the polygonal deflector while changing the scanning start position in the sub-scanning direction each time one scanning in the main scanning direction is completed. The laser beam printer is superior in the fast printing speed and high resolution of the printed image compared to other types of printers such as dot matrix impact printers. With recent increasing demand for higher resolution of images printed on print sheets, the demand for better resolution of picture elements (expressed by DPI (dot per inch)) of image forming apparatuses has also increased. Some laser printers offer a capability of selecting a desired picture element resolution from among a plurality of picture element resolutions according to the intended use of the output prints.
When the resolution of picture elements is increased, the number of revolutions of a polygon motor driving a polygonal deflector to be rotated has to be proportionally increased. For example, if the number of revolutions of the polygon motor when the picture element resolution is 200 dpi is 6000 rpm (revolutions per minute), for increasing the picture element resolution to 400 dpi, the number of revolutions of the polygon motor is required to be increased to 12000 rpm. In addition, unless the polygon motor reaches a state of revolving at a predetermined number of revolutions in a relatively short time after starting to revolve, such high speed feature of a laser beam printer is impaired. Therefore, the polygon motor is required to have a relatively short starting time to reach a state of revolving at a predetermined number of revolutions after starting to revolve, in addition to such features as stable revolutions at a relatively high speed, a long life time of bearings used in the motor such that the motor is suitable for use for a long time, and low electric consumption so that the heat emission is suppressed.
For shortening the starting time of a polygon motor, for example, the motor can be kept revolving at a high speed revolution even during a waiting period of an image forming apparatus in which the motor is used. However this causes the life time of the bearings of the motor to be decreased by a factor of several inverse squares of the number of revolutions of the polygon motor. Therefore, the polygon motor generally is stopped during the waiting period of the apparatus and is revolved when a printing operation starts.
Further, an operation of feeding a print sheet is generally started after a lock signal is detected indicating that the polygon motor has reached a state of revolving at a predetermined number of revolutions. However, for shortening the printing time, some image forming apparatuses are configured such that a print sheet is fed to reach an interim point of an image forming portion of the apparatus during the starting time of the polygon motor in order to reach a locked state at a predetermined number of revolutions. That is, the time the polygon motor takes to reach the locked state is estimated in advance with a controller of the image forming apparatus and the printing time is shortened by interleaving (1) the time required for feeding a first print sheet for a first printing job to the interim point of the image forming portion of the apparatus and (2) the time required for the polygon motor to reach the locked state.
However, the time required for a polygon motor to reach the locked state varies depending upon the apparatus in which the motor is used and depending upon the change of the apparatus according to the elapse of time. Therefore, the time to start feeding a first print sheet (i.e., the time. the first print sheet waits to be started to be fed after the revolution of a polygon motor is started) needs to be set such that the first sheet will not reach an image forming portion of the apparatus before the polygon motor reaches the locked state at a predetermined high speed. Accordingly, the timing generally is set such that a first printing operation, such as transferring a toner image to a print sheet from a photoconductor, is performed sufficiently after the polygon motor has been locked.
Also, a method has been proposed in which a polygon motor is revolved at a rate lower than a predetermined rate during a waiting period of the image forming apparatus and the time required for the polygon motor to reach a locked state for printing an image on a first print sheet is shortened. In this case also, the number of revolutions of the polygon motor in the waiting period generally is set higher than an optimum number of revolutions that enables the polygon motor to be locked, after a printing instruction has been received, at a timing closest to the timing when feeding of the first print sheet to the image forming portion is completed (i.e., when the time to feed the sheet to the image forming portion ends). Further, in image forming apparatuses, for coping with a variation in the time a polygon motor takes to reach a locked state resulting from variations in different types of apparatuses or from a change in the apparatus after the elapse of time, the operation of feeding a sheet is started well after the polygon motor has started to revolve.
However, in controlling the timing of starting the operation of feeding a sheet, the difference in the number of revolutions of the polygon motor according to the difference in the picture element resolution is not generally considered, and the timing is controlled simply based upon the longest time the polygon motor takes to reach the locked state, irrespective of the picture element resolution. Therefore, it occurs that the printing time for a first sheet is not shortened, depending upon the picture element resolution. Further, generally, the difference between (1) the time a sheet contained in one of a plurality of sheet cassettes takes to reach an image forming portion after having been started to be fed and (2) the time another sheet in another one of the plurality of sheets cassettes takes, generally is not considered in controlling the timing of feeding the respective sheets. Also, the timing is uniformly set based upon the longest time a sheet takes to reach the image forming portion after the sheet has started to be fed. Therefore, it occurs that the high speed revolution of the polygon motor is uselessly continued while waiting for a print sheet to reach the image forming portion, and thereby, the life time of bearings of the motor is decreased.
FIG. 14 is a timing chart illustrating an example of the operating timings of an image forming apparatus, starting from receipt of a printing instruction from a host computer to an exposure (image writing) of a photoconductor drum. The time when the signal is made high in an operation of receiving a printing instruction from a host computer indicates a time when the printing instruction from the host computer is received by a controller of the image forming apparatus. The time when the signal is made high in an operation of outputting a printing request from the controller indicates a time when the controller outputs the printing request to the printer engine of the image forming apparatus. The time when the signal is made high in an operation of a main motor is a time when the main motor starts to revolve to feed a sheet. The time when the signal is made high in an operation of a polygon motor is a time when the polygon motor starts to revolve to rotate a polygonal deflector. The time when the signal is made high in an operation of the sheet feeding clutch is the time when the sheet feeding clutch is connected to the main motor so as to transmit the revolution of the main motor to a feeding roller to feed the sheet. The time when the signal is made high in an operation of an exposure is a time when the exposure of a photoconductor drum is started.
In FIG. 14, an image bit-mapping time xe2x80x9caxe2x80x9d indicates the time the controller of the image forming apparatus takes to complete bit-mapping of image data after receiving the printing instruction for the image data from the host computer. A locking time xe2x80x9cbxe2x80x9d indicates the time the polygon motor takes to reach a stably locked state of revolving at a predetermined number of revolutions after having started to revolve in response to the printing request. A sheet feeding time xe2x80x9ccxe2x80x9d indicates the time required for a print sheet to reach a registration roller after the feeding clutch is connected to the main motor. A time xe2x80x9cdxe2x80x9d indicates the difference between the locking time xe2x80x9cbxe2x80x9d and the sheet feeding time xe2x80x9cc.xe2x80x9d A time xe2x80x9cexe2x80x9d indicates the difference between the time required for the print sheet to be fed from the registration roller to a transfer position where a toner image is transferred from the photoconductor drum to the print sheet and the time required for the photoconductor drum to rotate such that an exposed portion of the photoconductor drum moves from an exposure position to the transfer position. That is, the time xe2x80x9cexe2x80x9d delays the exposure time such that the exposure time matches the time of the print sheet to be fed to the transfer position from the registration roller.
As illustrated in FIG. 14, if a printing instruction is received by the controller of the image forming apparatus from the host computer, the controller outputs a printing request to the printer engine of the apparatus after the controller forms bit-map image data during the image bit-mapping time xe2x80x9ca.xe2x80x9d The printer engine starts the main motor and the polygon motor to rotate the polygonal deflector after the receipt of the printing request from the controller. The printer engine starts feeding a print sheet by connecting the feeding clutch to the main motor during the time xe2x80x9cd,xe2x80x9d which is the difference between the locking time xe2x80x9cbxe2x80x9d and the sheet feeding time xe2x80x9cc,xe2x80x9d after starting the main motor and the polygon motor.
However, because the sheet feeding time xe2x80x9ccxe2x80x9d of FIG. 14 is set such that the sheet reaches the image forming portion well before the polygon motor reaches a locked state, as described above, the polygon motor will not reach the state of revolving at a predetermined rate when the first sheet has reached the registration roller. Therefore, the first sheet waits at the registration roller until the polygon motor reaches the state of revolving at the predetermined rate. That is, when an image is formed on a first print sheet, a waiting time occurs for the polygon motor to reach a state of revolving at a predetermined number of revolutions.
Further, when (1) an image forming apparatus includes a plurality of sheet feeding cassettes and (2) a time for a sheet to reach an image forming position of the apparatus after having been started to be fed is longer than a time for a polygon motor to reach a state of revolving at a predetermined number of revolutions corresponding to a picture element resolution after receiving a request to revolve the polygon motor, the polygon motor reaches the state of revolving at the predetermined rate before a first sheet reaches a registration roller. As a result, the first sheet waits at the registration roller for the polygon motor to reach the state of revolving at the predetermined rate. Furthermore, if a controller of an image forming apparatus erroneously operates after a request to revolve a polygon motor is received and the polygon motor has started revolving, the polygon motor uselessly continues to revolve at a predetermined number of revolutions and as a result the life of bearings of the polygon motor is reduced.
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
The present invention provides a novel image forming apparatus and method that does not require a waiting time for a first print sheet, which is fed after a printing instruction from a host computer has been received. Since there is no waiting time for a polygon motor to reach a state of revolving at a predetermined number of revolutions for forming an image on the first sheet, the life of bearings of the polygon motor is lengthened by avoiding useless revolutions of the polygon motor at the predetermined rate for coping with the effects of variation and changes in the characteristics of the polygon motor over time.
The present invention also provides a novel image forming apparatus and method in which a polygon motor does not reach a state of revolving at a predetermined rate after a printing instruction from a host computer has been received before a first print sheet reaches a registration roller. The present invention also avoids useless revolution of the polygon motor at the predetermined rate for coping with variation in the time for the polygon motor to reach a locking state and changes in the characteristics of the polygon motor over time. Thereby, the life of the polygon motor is lengthened.
A novel image forming apparatus and method stops a polygon motor if a printer engine does not receive a printing request from a controller of the image forming apparatus within a predetermined time after the printer engine has received a polygon motor revolution request. In this manner, the present invention avoids revolving the polygon motor uselessly and lengthens the life of the bearings of the polygon motor.
Preferably, the image forming apparatus forms an image on a photoconductor by scanning light on the photoconductor and includes a printer engine having a polygon motor that can be controlled to revolve at an arbitrary rate. The apparatus also includes a deflector driven by the polygon motor to be rotated to deflect the light and a motor controller to control the revolution of the polygon motor. The apparatus further includes a controller to control the apparatus. The apparatus is connected to a host computer, and the polygon motor is controlled to stop revolving when the apparatus is in a waiting status. When the apparatus is set to form the image with a single picture element resolution, the controller outputs to the printer engine a request to revolve the polygon motor immediately after receiving a printing instruction to form the image from the host computer and a printing request to form the image after completing image processing for forming the image.
When the apparatus is set to form the image with a selected one of a plurality of picture element resolutions, the controller outputs to the printer engine the request to revolve the polygon motor. The request includes information regarding the selected picture element resolution for forming the image immediately after receiving the printing instruction from the host computer and the printing request to form the image after completing the image processing for forming the image.
Furthermore, the controller may output to the printer engine the request to revolve the polygon motor according to a picture element resolution previously set in the printer engine immediately after receiving the printing instruction from the host computer and the printing request. The printing request includes information regarding a revolution speed of the polygon motor corresponding to the selected picture element resolution for forming the image after completing the image processing for forming the image.
Further, the controller outputs to the printer engine the request to revolve the polygon motor a predetermined time after having outputted to the printer engine the printing request to form the image when (1) the image forming apparatus include a plurality of print sheet feeding devices to feed a print sheet therefrom and (2) a first time that is required for the print sheet to reach a registration roller to adjust a timing of the sheet to be fed to a transfer position of the printer engine where the image on the photoconductor is transferred to the sheet after the sheet has started to be fed is longer than a second time that is required for the polygon motor to reach a state of revolving at the rate corresponding to the selected picture element resolution for forming the image after the printer engine has received the request to revolve the polygon motor.
Furthermore, the printer engine may stop to revolve the polygon motor when the printer engine does not receive the printing request from the controller in a predetermined time after having (1) received the request to revolve the polygon motor from the controller and (2) started to revolve the polygon motor.